The fisherman was given a global fine of $1,000 on the two offences, which means he also has to pay additional mandatory penalties related to each of them.

Mandatory penalties are calculated at ten times the prescribed value of the fish that is the subject of any proven offence, either by weight or per fish.

Additional mandatory penalties of $9736.65 were applied to each of the two offences and based on the weight of the 954 mullet that were seized by Fisheries and Marine Officers.

In total, he will be required to pay $20,592.50 for the fine, the penalties and court costs.

The court was told that the accused's fishing boat licence states "no river or estuarine fishing" and he was therefore not entitled to fish the waters of the Vasse Estuary.

He had later admitted to setting nets in the Estuary on Wednesday 8 September, with the intention of catching mullet for bait, but (contrary to a legal requirement to remain within 100 metres of nets set for fishing) he had left the area and returned to Bunbury.

Police and Fisheries and Marine Officers had stopped the offender's vehicle, while he was towing his boat, around 6 o'clock the next morning on Layman Road in Wonnerup, after an overnight surveillance operation.

Supervising Fisheries and Marine Officer Kevin Johnson said the fine and penalties were significant and reflected the seriousness of the offences.

"The Department of Fisheries had responded to information of potential illegal fishing and with the help of Police we were able to set up a surveillance operation that netted the commercial fisher," Mr Johnson said.

"It is another example of community vigilance and joint agency efforts, to ensure that any person fishing in the Vasse Estuary complies with the rules."

Anyone with information about illegal fishing should call FISHWATCH on 1800 815 507.